It has been proposed to coat fertilizers with various polymers and inorganic materials. For example, Japanese Patent Publication Nos. 28927/1965 and 13681/1967 disclose various coating materials, but they have difficulties in controlling the dissolving-out rate of fertilizer nutrients.
U.S. Pat. No. 4,369,055 and Japanese Patent Application Laid-Open No. 167197/1980 have tried to facilitate degradation of coating materials while maintaining the function for controlling the dissolving-out rate by dispersing inorganic powders such as sulfur and talc in a low molecular olefin polymer. However, the disclosed coating materials are not completely degraded and they remain in soil.
Further, U.S. Pat. Nos. 5,176,734 and 5,206,341 and Japanese Patent Application Laid-Open No. 146492/1991 propose to coat a fertilizer with a biodegradable polymer. They only disclose to coat the fertilizer with a biodegradable coating material in a single layer. The single layer coating has a difficulty in controlling both the dissolving-out rate of fertilizer nutrients and the biodegradability at the same time.
Japanese Patent Application Laid-Open No. 97561/1993 discloses a three layer coating prepared by using one type of biodegradable film and a water-soluble resin. It also has a difficulty in controlling both the dissolving-out rate and the biodegradability at the same time. The application discloses that a thickness of the coating material is 500 to 2,000 .mu.m. This invites high cost so that the coating material is not suitable for practical use.
The problem addressed by the present invention is to provide a granular fertilizer with a multilayer coating for agriculture and gardening, in which the dissolving-out rate and dissolving-out pattern of fertilizer nutrients are controllable and also the coating material does not remain in soil or water after the fertilizer nutrients are dissolved out.